Ostracod Evolution and Depositional Characteristics of the Cretaceous Nenjiang Formation in the Songliao Basin, China

Abstract

In the Cretaceous Songliao Basin, the Nenjiang Formation is the best-developed and is overlain by the Sifangtai Formation and underlain by the Yaojia Formation. The Nenjiang Formation is more than 1000 m thick and forms a great area of dark mudstone and sandstone, where the oil-bearing beds of the Heidimiao and Shaertu reservoirs Sa-l-Zu and Sa-O-Zu are located. The formation changes little both in lithology and thickness and mainly consists of grey-dark mudstones, shales and oil-shales, grey-green mudstones, intercalating grey, grey-white fine silt sandstones and red mudstones at the upper part of the formation. Upwards, the Nenjiang Formation becomes finer in grain size and richer in fossils. Overall, the Nenjiang Formation can be divided into five successive members based on the vertical changes in lithology and fossil ostracods. During recent years, secondary exploration in the Songliao Basin has taken place. The Hiedimiao reservoir of the Nenjiang Formation is of great interest to the oil industry and as a result, palaeontologists have also done a great deal of work on it, especially detailed work on ostracods.